This invention relates to an isolation bearing for structures and attachments and, in particular, an isolation bearing having several degrees of kinematic freedom.
It is known in the art to isolate a structure from shock and vibration by inserting isolation bearings between the structure and its foundation. One purpose of isolation bearings is to damp vibration coupled to a structure from external sources such as seismic disturbances or vehicular traffic. Another purpose of isolation bearings is to reduce the coupling of dynamic forces to a structure, permitting structures to be constructed to withstand smaller forces than predicted for the site.
An early form of isolation bearing had horizontal plates separated by several layers of elastomer. Vibration was absorbed in the elastomer. Another form of isolation relied on friction between plates in contact to damp vibration. This has some disadvantage because the static friction coefficient between the plates is higher than the sliding friction coefficient. Therefore, a higher force is required to mobilize the energy absorption (hysteretic damping) in this device than the force needed to maintain it.
There is a dichotomy in structural bearings: isolation versus anchor. A highly isolated structure is undesirable because there is too little restraint on its movement. A firmly anchored structure is subject to all ambient forces. It is difficult to provide a bearing which anchors a structure and isolates it.
The isolation bearing disclosed in U.S. Pat. No. 4,910,930 uses a plurality of horizontal, metal plates separated by layers of high damping rubber. Coiled steel rods limit the lateral motion of the plates. High damping rubber is less resilient than low damping rubber, i.e. the energy used to deform or strain the rubber is partially absorbed and converted into heat. In U.S. Pat. No. 4,823,522, a plurality of spaced, vertical metal plates plastically deform to absorb energy applied to the structure. In all of these devices, the number of degrees of freedom is limited, e.g. permitting movement along one or two axes of the bearing.
U.S. Pat. No. 4,727,695 discloses an isolation bearing using opposing plates having interlocking cross-sectional configurations to form a keyed shear and uplift proof element. An elastomer layer between the plates provides shock and vibration isolation and energy dissipation. Although the plates are interlocked, the bearing has several degrees of kinematic freedom and restrains forces and moments in several ways.
Although a bearing under a column of a building does not need more than three degrees of freedom, there are many applications for isolation bearings which require up to six degrees of freedom. To give but two examples: a skewed, elevated roadway on a curved alignment and a ramp connecting a floating dock to a fixed platform. An elevated roadway uses a plurality of sections of roadway supported on piers or columns. The roadway is subjected to forces from many sources, e.g. vehicular traffic, wind, thermal expansion, and seismic disturbances. A bearing on a pier supporting the middle of a section of roadway can twist as adjacent sections shift in an uncoordinated fashion, forcing the top plate of a bearing to rotate or twist with respect to the bottom plate about a vertical axis. In addition to twist, the isolation bearings connecting a ramp to a floating dock must also accommodate rotation, i.e. a tilting of the top plate out of a horizontal plane, as the floating dock rises and falls with the tide.
In view of the foregoing, it is therefore an object of the invention to provide an isolation bearing having more degrees of kinematic freedom than bearings of the prior art with comparable restraining force capacities.
Another object of the invention is to provide an isolation bearing in which stress is uniformly distributed in metal components of the bearing.
A further object of the invention is to provide an isolation bearing which can be made from thinner metal components than those of the prior art, without loss of load capacity or energy dissipating capacity.
Another object of the invention is to provide an isolation bearing having a multi-mode, progressive, energy dissipating characteristic.
A further object of the invention is to dissipate energy in an isolation bearing by deformation of the elastomer and of the metal components and by friction between metal components.
Another object of the invention is to provide an isolation bearing in which the metal components are heat sinks for the elastomer.
A further object of the invention is to provide an isolation bearing in which metal components control the bulge of the elastomer.
A further object of the invention is to provide an isolation bearing in which the elastomer is prevented from slipping along a metal component even though the elastomer no longer adheres to the metal component.
Another object of the invention is to provide an isolation bearing which can be easily adapted to a wide variety of applications.
A further object of the invention is to increase the vertical load carrying capacity of a bearing by controlling elastomer bulge without losing horizontal isolation capacity.